Production of electrolytic tinplate



United States Patent.

3,334,030 PRODUCTION OF ELECTROLYTIC TINPLATE George K. Notman, Pittsburgh, Pa., assignor to Jones & Laughlin Steel Corporation, Pittsburgh, Pa., a corporation of Pennsylvania No Drawing. Filed Apr. 10, 1964, Ser. No. 358,944

6 Claims. (Cl. 204-37) This invention is concerned with the production of electrolytic tinplate of improved corrosion resistance for use in food packs. It is more particularly concerned with a two stage process which insures the presence of interfacial iron-tin alloy layer of controlled thickness.

The great bulk of tinplate used today is provided with its tin coating by electrolytic tinning processes. These processes permit the plating of a relatively thin tin coating which is relatively uniform in thickness. Because the thickness of tin on conventional tinplate is a small fraction of an inch, the amount of tin coating is more conveniently expressed in terms of its weight in pounds for 3 a base box of tinplate. The term base box is a measure of area or surface, and amounts to 31,360 square inches. Large amounts of electrolytic tinplate are made with coating weights on the order of /2 pound of tin per base box of tinplate. v

With the thin tin coatings now applied, the electrolytic tinplate used for food packs must be tested carefully to determine its resistance to corrosion by various food products. In the past the determination of corrosion resistance of tinplate has been a rather tedious process, but in recent years it has been found that the corrosion resistance of tinplate used for food packs can be determined by a galvanic test, known as the alloy-tin couple test. The test consists of stripping the tin from a sample of tinplate down to the tin-iron alloy surface and measuring the current density developed by a galvanic couple comprising a pure tin electrode and the sample immersed in grapefruit juice containing 100 p.p.m. of soluble stannous tin, at a temperature of 79 F. The current density after 20 hours is measured in microamperes per square centimeter, and the figures are referred to as ATC values. Low ATC values indicate good corrosion resistance, whereas high ATC values represent poor corrosion resistance. The ATC test is described in the paper The Alloy- Tin Couple Test-A New Research Tool by G. G. Kamm, A. R. Willey, R. E. Beese and J. L. Krickl, published in Corrosion, volume 17, Febraury 1961, pages 106-112.

Commercial electrolytic tinplate as produced under varying conditions has ATC values ranging from 0.01 up to perhaps 0.5. The ATC value of 0.07 has been arbitrarily selected as representing superior quality tinplate, although, as I have mentioned, it is possible to produce tinplate having lower ATC values.

Tin may be electro-deposited upon a steel base using either an alkaline or an acid electrolyte. My invention is particularly directed toward tinplate produced by acid processes. One acid electrolyte commonly used is that of Us. Patent 2,407,579 issued to'E. W. Schweikher on Sept. 10, 1946. That electrolyte includes stannous chloride and an alkali fluoride, and the process employing it is commonly known as the halogen process.

It is an object of my invention to provide a process of manufacturing electrolytic tinplate of conventional coat ing thickness, but of superior corrosion resistance. It is another object to provide such a process in which the irontin alloy layer is of a character to insure superior corrosion resistance. It is another object to provide a process as above described which permits control of the iron-tin alloy layer thickness independent of the tin coating thickness. Other objects of my invention will appear in the course of the following description thereof.

My process includes, briefly, the deposition on the steel of a thin layer of commingled iron and tin. This layer is preferably deposited electrolytically. A layer of tin is next electro-deposited on top of the bimetallic layer. The article so plated is then heat treated at a temperature and for a time suflicient to bring about alloying of the iron and tin first deposited.

In an embodiment of my invention presently preferred by me, the steel is cleaned by conventional solvent and/ or electrolytic methods, and is given a light pickle in an aqueous solution of 1% hydrochloric acid. The steel is then electroplated with tin and iron in an electrolyte of the following typical composition:

The pH of the solution is adjusted to a value of about 2.5 by the addition of ammonium hydroxide. The strip is plated with tin anodes at a current density of about 24 amps. per square foot with the electrolyte at a temperature of 65 C. Over the coating so obtained tin in the amount of /2 pound per base box is deposited from a conventional halogen bath as disclosed in the Schweikher patent before mentioned. The tinned strip is then brightened by momentarily heating it to a temperature between about 550 and 600 F. and quenching it in Water.

Seven samples of material were plated as above described. The plating time for the deposition of iron and tin was varied from 3 seconds to seconds, with the results which are shown in Table I.

TABLE I Sample Plating Time LbJbb. Final Alloy I ATC- Micro- (secs.) Fe/Sn Wt. (1b./bb.) Amps/em.

It is apparent from the table that the ATC values for sample numbers 4 and 5 are markedly superior to those of the other samples. Sample 4 was plated with .11 pound per base box of commingled iron and tin and sample 5 was plated with about .17 pound per base box of iron and tin. The results show that the weight of the commingled iron-tin layer should be between about .1 and .2 pound per base box for maximum corrosion resistance of the tinplate as measured by ATC values.

The electrolyte I have described is well suited to my process, because iron and tin can be deposited therefrom at speeds comparable to those of electro-deposition of tin from conventional halogen electrolytes, and because it is compatible with halogen electrolytes. It is my belief, however, that a product having superior corrosion resistance can be produced by my process regardless of the specific method of depositing the layer of commingled iron and tin. Although I do not know the mechanism which brings about the improved corrosion resistance of tinplate produced by my process, it is my theory that the improvement results from the presence of an effective iron-tin alnism, but, as the time is quite short during which the tinplate is at temperatures high enough for alloyin-g, the amount of alloy formed by conventional practices must be limited. By providing larger amounts of iron and tin in contact with each other at the interface, I make possible the subsequent formation of more iron-tin alloy or, conceivably, of an iron-tin alloy layer in a form better adapted for corrosion prevention.

I have described my process as making use of tin anodes. Prolonged operation of my electrolyte with tin anodes results in depletion of its iron content which must be brought up to the required level by additions of ferrous chloride. Alternatively, I employ anodes made of a tin-iron alloy or a combination of iron anodes and tin anodes arranged side by side in the bath.

I claim:

1. The process of providing a ferrous base metal with a corrosion resistant tin coating comprising depositing on the ferrous base metal a first coating comprising commingled iron and tin, then electroplating the coated metal with a second coating of tin, then heating the coated metal to a temperature and for a time sufiicient to alloy the iron and tin of thefirst coating.

2. The process of providing a ferrous base metal with a corrosion resistant tin coating comprising electroplating on the base metal a first coating comprising commingled iron and tin from a bath containing a compound of iron soluble therein and a compound of tin soluble therein, then electroplating on the coated metal a second coating of tin, then heating the coated metal to a temperature above the melting point of tin for a time sufficient to alloy the iron and tin of the first coating and to fuse momentarily the tin of the second coating and then quenching the coated metal.

3. The process of claim 2 in which the first coating has a weight between about .1 and .2 pound per base box of tinplate.

4. The process of claim 2 in which the bath is aqueous, the compound of iron is ferrous chloride, the compound of tin is stannous chloride, the anode is tin and the ferrous base metal is the cathode.

5. The process of claim 4 in which the bath contains an alkali fluoride.

6. The process of claim 4 in which the anode comprises iron and tin.

References Cited UNITED STATES PATENTS 2,069,658 2/1937 Rcnkin 29--196.4 2,266,330 6/1941 Nachtman 204-28 3,260,580 7/1966 Karnm et al. 29-196.4

FOREIGN PATENTS 662,961 5/ 1963 Canada.

JOHN H. MACK, Primary Examiner.

W. VAN SISE, Assistant Examiner. 

1. THE PROCESS OF PROVIDING A FERROUS BASE METAL WITH A CORROSION RESISTANT TIN COATING COMPRISING DEPOSITING ON THE FERROUS BASE METAL A FIRST COATING COMPRISING COMMINGLED IRON AND TIN, THEN ELECTROPLATING THE COATED METAL WITH A SECOND COATING OF TIN, THEN HEATING THE COATED METAL TO A TEMPERATURE AND FOR A TIME SUFFICIENT TO ALLOY THE IRON AND TIN OF THE FIRST COATING. 